The art of making prepregs is extremely well known. Prepregs typically comprises fabrics (woven and/or nonwoven) and/or continuous filaments of high performance materials such as those having a melting point (T.sub.m) or glass transition temperature (T.sub.g) of at least about 130.degree. C. Suitable filaments include, by way of example, glass filaments, carbon and graphite filaments, aromatic polyamides (polyphenyleneterephthalamide) such as Kelvar.RTM., metal fibers such as aluminum, steel and tungsten, boron fibers, and the like.
The filaments are typically bundled into tows and the tows are assembled and spread out into a relatively thin sheet which is either coated or impregnated by the matrix resin. The matrix resin is the typical high performance thermosetting or thermosettable resins. The combination of the filament and the resin results in a prepreg suitable for forming an advanced composite structure. The resin may be any of those thermosetting or thermosettable resins employed in the manufacture of advanced composites. The most common class of resins are the epoxy resins. They are frequently based, inter alia, on one or more of diglycidyl ethers of bisphenol A (2,2-bis(4-hydroxyphenyl)propane) or sym-tris(4-hydroxyphenyl)propane, tris(4-hydroxyphenyl)methane, their polyepoxide condensation products, cycloaliphatic epoxides, epoxy-modified novolacs (phenol-formaldehyde resins) and the expoxides derived from the reaction of epichlorohydrin with analine, o-, m- or p-aminophenol, and methylene dianaline. Illustrative resins are epoxies curing at 350.degree. F. (177.degree. C.) and 250.degree. F. (121.degree. C.). Other thermosetting or thermosettable resins include the bismaleimide (BMI), phenolic, polyester (especially the unsaturated polyester resins typically used in SMC production), PMR-15 polyimide and acetylene terminated resins have been found suitable.
There has been a need to make epoxy composites tougher. This point is addressed by Chu, et al., U.S. Pat. No. 4,656,208, patented Apr. 7, 1987, and, in part, by Jabloner, et al., U.S. Pat. No. 4,656,207, patented Apr. 7, 1987, in the following manner:
"One approach in making epoxy composites tougher has been to introduce functionally terminated rubbery polymers into the epoxy resin formulations. The thermosets resulting from these formulations, while having increased toughness, have reduced modulus. PA1 "Another approach has been to incorporate engineering thermoplastics into the epoxy resin formulation. Various therrmoplastics have been suggested and the use of polyethersulfone as the thermoplastic modifier for epoxy resin formulations was studied by C. B. Bucknall et al and is discussed in the British Polymer Journal, Vol. 15, March 1983 at pages 71 to 75. Bucknall et al's studies were carried out on cured epoxy-polyethersulfone blends prepared from trifunctional and/or tetrafunctional aromatic epoxides, diaminodiphenylsulfone or dicyandiamide hardener and various amounts of Victrex 100P manufactured by ICI Ltd., said to be a relatively low molecular weight grade of polyethersulfone. The studies showed that phase separation occurred in certain of the cured epoxy-polyethersulfone blends and that some of the cured blends exhibited distinct nodular morphological features. Analyses indicated that the polyethersulfone was concentrated in the nodules and Bucknall et al surmised that the nodules were not formed by polyethersulfone alone but by a crosslinked epoxypolyethersulfone copolymer. Bucknall et al found no clear correlation between composition and mechanical properties such as elastic modulus, fracture toughness and creep of the cured blends and concluded that the addition of the polyethersulfone had little effect on the fracture toughness of the resin mixture, irrespective of the degree of phase separation or the morphology. PA1 "Yet another approach for improving the mechanical properties of cured epoxy resins is described in U.S. Pat. No. 4,330,659 to King et al. King et al disclose using as the hardener for epoxy resins the reaction product of diaminodipheny sulfone with diglycidyl ethers of polyhydric phenols. The cured resins prepared from mixtures of a `modified` hardener obtained by adducting the coreaction product of the diglycidyl ether of bisphenol A and additional bisphenol A with diaminodiphenylsulfone, and tetraglycidylated methylenedianiline are reported to have increased impact strength and toughness as compared with the cured resins obtained using unmodified diaminodiphenylsulfone as the hardener."
The Chu, et al. and Jobloner, et al. patents purport to differentiate by using as an amine hardener, an amine end-blocked oligomer of the conventional bis-A based polysulfone. The oligomer is characterized as having a number average moleculer weight of between about 2,000 and about 10,000.
Diamant, et al., Development Of Resins For Damage Tolerant Composites--A Systematic Approach, 29th National SAMPE Symposium, Apr. 3-5, 1984, pages 422-436, disclose the use in composite formulations of blends of TGMDA/DDS.sup.1 epoxy resins and such thermoplastics as the polyethersulfones ("PES") and polyetherimides (PEI) of the repeating unit formulae: EQU --[--O--Ph--SO.sub.2 --Ph--].sub.n --
and EQU --[--O--Ph--O--Ph'(CO).sub.2 &gt;N--Ph--N&lt;(CO).sub.2 Ph'--].sub.n --
wherein Ph is 1,4-phenylene, Ph' is phenylene in which the carbonyls (CO) are carbon bonded to ring carbons thereof that are ortho as to each other and each ether oxygen (O) is bonded to a ring carbon thereof that is para to one of the ring carbons to which is bonded one of the carbonyls, n has a value sufficient to provide a reduced viscosity of at least about 0.4, as prepreg matrix resins. The polyethersulfone is available as Victrex.RTM. PES from ICI Advanced Materials, Wilmington, Del. 19897, and the polyetherimide is available as Ultem.RTM. from General Electric Company Company, Plastics Business Group, Pittsfield, Mass. According to Diamant, et al., the thermoplastic polymers make the resin more toughenable. FNT 1. "TGMDA" is the abbreviation for tetraglycidyl methylene dianiline or bis (N,N-diglycidyl-4-aminophenyl)methane and "DDS" is the abbreviation for bis(4-aminophenyl)sulfone.
Leslie, et al., Chemtech, July 1975, pages 426-432, describe the combination of epoxy resins and PES for upgrading the temperature performance characteristics of epoxy resin adhesives.
St. Clair, et al., U.S. Pat. No. 4,510,277, disclose the use of Cr(AcAc).sub.3 , i.e, tris(acetylacetonate)chromium (III), in amounts ranging from 1-13% by weight, as additives to TGMDA/DDS resin systems and to DGEBA (bis-2,2-(4-glycidylphenyl)propane and condensation products thereof), for the purpose of improving the moisture resistance of the systems.
A silica ceramic whisker called Xevex.RTM., sold by J.M. Huber Corporation, Borger, Tex. 79008, has been recommended as a reinforcing fiber for epoxy resins. The amounts recommended in epoxy resins are at conventional filler loading levels of about 40-50 part per hundred parts of the resin.
It has been determined that a layered prepreg formulation comprising continuous filament fibers such as carbon and/or graphite fiber arranged in the form of a prepreg structure, and/or a fabric of the same fibers (woven and/or nonwoven), and impregnated with a matrix resin containing an epoxy resin and an engineering thermoplastic polymer can be provided with capacity to produce laminated structures having enhanced impact resistance by incorporating at the surfaces of the prepreg a thin layer, less than about 80 microns in thickness, of small fibers, fibrids or particles made of polymers having at least one of a T.sub.m or T.sub.g greater than about 300.degree. C. The invention does not depend on the method for making the prepreg. Many diffferent kinds of prepregs may be employed without deviating from the invention. The invention encompasses also articles and compositions which provide for an enhanced toughened laminate structures and laminate structures with enhanced compressive strength.